1. Field of the Invention
The present invention relates to laser resonators and more particularly to the generation of high power, high beam quality laser beams.
2. Description of the Related Art
The impact that semiconductor lasers have had on the development of solid-state lasers is evident in high-average-power solid-state laser systems, which are finding increasingly wide application in the field of material processing. High-average-power diode-pumped solid-state lasers (HAP DPSSLs.) are best characterized as being performance limited by thermal management issues associated with the solid state laser crystal. These thermal management issues can manifest themselves in several different forms. Examples include aberration of beam quality due to thermally induced refractive index variations in the crystal, and in extreme cases, fracture of the laser crystal due to the buildup of excessive stress in the crystal. The most commonly used ion/host crystal combination is Nd3+:YAG. Of all the well-developed laser crystals that can be grown with rare-earth impurities, YAG is the most robust in the thermal sense; i.e., it has the highest thermal conductivity and the highest resistance to fracture. For these reasons, YAG is the crystalline host of choice for HAP DPSSLs.
Recent developments in laser diode array technologies have resulted in Lawrence Livermore National Laboratory (LLNL) pursuing a diode array pump technology that enables pump power to be delivered at much higher irradiances than was previously possible. Many rare-earth ion based HAP DPSSLs are only practical if pumped at higher irradiances than the several kW/cm2 that is possible with conventional scaled diode array technologies. Thus, this direction has opened a path to HAP DPSSLs that rely on ions other than Nd3+ as their lasant. LLNL has demonstrated a HAP Yb:YAG DPSSL which generates a very high average brightness. This LLNL design uses a dual rod configuration consisting of two, 5 cm long by 2 mm diameter laser rods with birefringence compensation, and has achieved 1080 W of cw output with an M2 value of 13.5 at an optical-to-optical conversion efficiency of 27.5%.
The approach that LLNL has followed to achieve good beam quality at high-average power relies on designing the laser resonators to run with large diameter fundamental modes in the presence of strong thermal lensing. The laser rod itself acts as a spatial filter, limiting the number of transverse modes beyond the fundamental, which are supported by the cavity without incurring large diffractive losses. However, in single rod lasers, stress-induced birefringence and bifocussing place a limit on the TEM00 mode size that can be produced. As a result the stability zones for the radially polarized light and the tangentially polarized light can pull apart at high thermal loading due to the difference between the stress in the radial and tangential directions in the rod. This leaves little possibility for stable mode operation of the laser cavity. One technique, which was originally demonstrated in 1970 can, to a great extent, eliminate birefringence and bifocussing effects by using two identically loaded rods separated by a 90xc2x0 rotator. There have been several recent reports of groups using this birefringence compensation technique successfully with diode-pumped Nd:YAG lasers. LLNL has used the same approach to compensate for birefringence and bifocussing in their Yb:YAG laser.
For pump powers greater than 3 kW LLNL inserted a negative lens between the two laser rods to maintain resonator stability. This lens was housed in the same fixture used to hold the 90xc2x0 rotator. The lens was varied as a function of the applied pump power and chosen so as to maintain a near constant TEM00 beam waist in the presence of the strong thermal lens in the laser rods. The TEM00 beam waist is held constant by the choice of a compensating negative lens. Therefore, it would be expected that the external beam quality of the developed laser radiation would also be nearly constant.
This technology has since undergone further development at The Boeing Company. Repeated laboratory experience in setting up, aligning and operating the laser has shown that the thermal effects represent a significant hurdle to further development. In particular, thermal damage has occurred in both the lens and the rotator, requiring replacement of these components or laser operation at power levels low enough to minimize damage.
U.S. Pat. No. 3,748,015, issued to A. Offner, discloses an optical device that is now known as an xe2x80x9cOffner relayxe2x80x9d. Such a device has been typically used, heretofore, in photolithography. The Offner relay was originally used as a mask aligner for projecting a telecentric image of a mask onto a semiconductor wafer. It is well known within the photolithography field, as is its ability to create high-quality images. The Offner relay consists of two concentric spherical mirrors, a primary mirror whose surface is concave and a secondary mirror whose surface is convex. The primary mirror reflects the image to the secondary mirror, which in turn reflects the image back to the primary mirror.
In a broad aspect, the laser resonator system of the present invention includes a first reflector; a second reflector; at least a first gain medium; a second gain medium; and, an Offner relay system. The first gain medium and the second gain medium are operatively positioned relative to the first and the second reflectors. The first and second gain media generate a laser beam. The Offner relay system is operatively positioned between the first and the second gain media for relaying the laser beam between the first and the second gain media.
Utiilization of an Offner relay system inside a laser cavity minimizes or obviates the need for refractive elements in this resonator. This accomplishes several things:
Light is reflected away from the Offner relay system to form an image of the object that is free of all first-order aberrations; i.e., there is no spherical aberration, coma, astigmatism, field curvature, or distortion. Although some aberrations are introduced when the light first reflects off a primary mirror of the Offner relay system, these are exactly cancelled by aberrations introduced by a second reflection off the primary mirror, so that the Offner relay system, itself, introduces no first-order aberrations to the image. Since the relay has no refractive components, there is the additional benefit that no chromatic aberration is added to the image. The result is excellent image quality across the entire field.
It reduces the distortions in the beam caused by heat-induced substrate deformation.
It is easier to cool mirrors than lenses, so the possibility of greater laser power is created.
The Offner relay system introduces a minimum of new aberrations, so the focus it produces is superior than that of a lens.
The above factors result in a very stable, high quality, and high power output beam.